1. Field of the Invention
The present invention relates to wide angle lens systems, and more particularly to a wide angle lens system for use in copying machines of the full exposure type wherein the entire surface of a document is exposed to light at a time to project the light reflected from the surface on a photosensitive member.
2. Description of the Related Art
The document exposure systems for copying machines heretofore known are those of the slit exposure type and those of the full exposure type. With the slit exposure type, the photosensitive member is continuously exposed to an image of the document in the form of a slit using a projection lens system which can be relatively narrow in the angle of view, whereas the machines of this type have the drawback of being relatively low in copying speed since the entire surface of the document must be exposed to light by a scanning movement. On the other hand, the full exposure type is adapted for a high-speed copying operation since the document need not be scanned, but the projection lens system must have a very wide angle of view for exposing the photosensitive member to the entire document image at a time.
Stated more specifically with reference to FIG. 1 showing the full exposure system for copying machines, an image of a document 1 on a document support table 2 is projected on a photosensitive belt 3 by a projection lens system 4. Because of this arrangement, the conjugate distance L between the document 1 and the photosensitive belt 3 exerts a great influence on the size of the entire copying machine. To make the copying machine compact, there is a need to decrease the conjugate distance L and therefore to give a very wide angle of view to the projection lens system 4. It is desired that the projection lens system 4 to be used be 60 degrees or greater in the angle of view 2.omega..